1. Field of the Invention
The present invention relates to methods of preparing .alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as ".alpha.-APM") or its hydrochloride salt (hereinafter referred to as ".alpha.-APM.HCl"). More particularly, it relates to methods of preparing .alpha.-APM or .alpha.-APM.HCl by solid-liquid separation of an aqueous suspension of the salt.
2. Discussion of the Background
.alpha.-APM is a low-calorie dipeptide diet sweetener having a sweetness which is about 200 times that of sucrose (cane sugar). It is expected that the world demand for this product will exceed 10,000 tons before 1995. The hydrochloride salt of .alpha.-APM, .alpha.-APM.HCl, is an important intermediate for the preparation of .alpha.-APM.
The following methods are examples of known industrial methods for the production of .alpha.-APM:
(1) a method of obtaining .alpha.-APM wherein an N-protected aspartic acid anhydride and a phenylalanine methyl ester are condensed in an organic solvent and the protective group is removed from the product (see U.S. Pat. No. 3,786,039 incorporated herein by reference); PA1 (2) a method of obtaining .alpha.-APM in which .alpha.-L-aspartyl-L-phenylalanine is methyl-esterified in a mixed solvent comprising water, methanol and hydrochloric acid, to obtain .alpha.-APM.HCl, which is then neutralized to obtain .alpha.-APM (see Japanese Patent Application Laid-Open No. 53-82752); and PA1 (3) a method of obtaining .alpha.-APM in which an N-protected aspartic acid anhydride and a phenylalanine methyl ester are condensed in the presence of an enzyme, and the protective group is subsequently removed from the product (see Japanese Patent Publication No. 55-135595).
In above synthetic method (1), the .beta.-isomer (i.e., .beta.-L-aspartyl-L-phenylalanine methyl ester (.beta.-APM)) is produced as a side product. As a means for selectively removing impurities, including this .beta.-isomer, there is known a purification method (4) in which an .alpha.-APM product containing impurities is brought into contact with a hydrohalogenic acid and then subjected to solid-liquid separation so as to isolate .alpha.-APM as its hydrohalide salt.
When considering the industrial scale production of .alpha.-APM to meet the trend of present consumption demands, chemical methods are presently favored from the viewpoint of manufacturing cost. In such cases, esterification of the product is often achieved via its hydrochloride salt as in above method (2), or after formation of .alpha.-APM, the .alpha.-APM is often made into its hydrohalide salt (.alpha.-APM.Hal), such as hydrochloride salt to purify it as in above method (4).
To obtain .alpha.-APM from its hydrohalide salt, such as its hydrochloride salt, the .alpha.-APM.Hal is dissolved or suspended in an aqueous medium and the resulting solution or suspension neutralized through the addition of a base. This is followed by cooling the neutralized liquid to yield a suspension of .alpha.-APM. The resulting suspension of .alpha.-APM is then subjected to solid-liquid separation. In general, a centrifugal separator is used for such solid-liquid separation
When .alpha.-APM.HCl, which is a useful intermediate in the preparation of .alpha.-APM, is separated by a method such as above methods (2) or (4), there is a high probability of product contamination by undesirable by-products. In above method (4), after removal of the protective group, the reaction mixture still contains various impurities such as .beta.-APM, in addition to .alpha.-APM. In above method (2), the .alpha.-L-aspartyl-L-phenylalanine (.alpha.-AP) used as the starting material for methyl-esterification, most often contains impurities, such as .beta.-APM as a by-product from the previous step. In these cases, however, a high-purity .alpha.-APM.HCl product can be obtained by selectively precipitating .alpha.-APM.HCl on the basis of its difference in solubility, followed by solid-liquid separation of the suspension. For such solid-liquid separation, in general, a centrifugal separator is used.
For separating a suspension of .alpha.-APM and .alpha.-APM.HCl, the mother liquor which remains adhered to the cake must be removed by washing the cake. As the washing procedure, substitution washing is suitable because of its simplicity. However, where separation of the suspension is performed with a centrifugal separator, as is generally done, there is the significant problem that the amount of washing liquid necessary for the washing is fairly large.
When one considers (a) the large amount of impurities which can be present in the mother liquor, (b) the high purity required of .alpha.-APM as a commercial product and (c) the role of the .alpha.-APM.HCl as an intermediate for preparing .alpha.-APM, the .alpha.-APM and .alpha.-APM.HCl crystals have to be washed to a level sufficient to achieve the highest purity attainable. However, when the amount of washing liquid is increased, not only does the cost of the process increase because of the increased cost of the washing liquid, but there is also necessarily a decrease in product yield due to increasing loss of .alpha.-APM or its hydrochloride salt and other valuable compounds from the cake due to their dissolution into the washing liquid. Even when recovery of product dissolved in the washing liquid is taken into consideration, the operation is very costly and difficult because of the large amounts of liquid processed and the low concentration of compounds dissolved therein.
Where a centrifugal separator is used and it is desired to increase the degree of dehydration, the separating operation must be effected in a batchwise manner. If the feeding of liquid, separation, washing, dehydration, and the removal of the cake are carried out as a series of operations, there is the additional problem that the requisite processing plant must be large-scaled.
In addition, the operation itself is also complicated. Besides the complication of having to carry out the separation several times, there is another problem in that the cake which adheres to the filter cloth must be scraped off by hand. This additional process step requires an increase in the amount of labor (number of workers) needed for the operation, and the time and number of steps required for processing increase, resulting in overall increased cost and overall decreased yield. In light of the considerable commercial demand for .alpha.-APM there is a clear need for notably improved processes free of the above disadvantages.